Another (possibly) watery moon in the outer solar system

Mimas is the closest moon to Saturn, and with a radius of just 198 kilometers (123 miles), is the smallest of the planet’s major satellites. Mimas has fascinated planetary scientists ever since it was first seen up close by Voyager 1, due to its resemblance to the “Death Star” from Star Wars. The moon’s appearance is dominated by a major asteroid impact crater called Herschel, which is more than 80 miles across, with a four-mile-high central peak.

Using the Narrow Angle Camera on the Cassini spacecraft, they found that Mimas wobbles back and forth while orbiting Saturn, which would not be expected if it were a rocky ice body with a small round core, as previously expected. Either Mimas has (for some reason) an oddly-shaped core, or (more likely) there is an ocean below its rigid ice shell, at a depth of 24 to 31 kilometers (15 to 19 miles). Since Mimas has quite an eccentric (elliptical) orbit, the idea advanced by Tajeddine et al. is that tidal heating could have formed a liquid ocean in the past.

Interestingly, most scientists used to think that Enceladus, a slightly larger moon of Saturn, would be way too small to retain liquid water underneath the ice. After this report, Mimas may represent a new lower limit for such an ocean. If this is correct, then ice-covered oceans might be common in the outer Solar System, and perhaps in the universe at large. In fact, they may be much more common than the “naked” ocean found on Earth. Of course, the presence of an ocean does not mean life, but it could mean habitable conditions and a niche that could be colonized if life could get there.

This may particularly be true in the outer Solar System, which is rich in organic compounds as well as liquids and gases that can be utilized by life. In the case of Mimas, however, no exchange with the surface seems to take place. This is unlike Enceladus, where geysers deposit fresh ice on the surface near the South Pole. What we have on Mimas is probably “fossilized” water that has remained in the moon’s interior for a very long time.

Sending a spacecraft to analyze the composition of the Mimas ocean would certainly be intriguing. Could water have been locked inside the moon for billions of years? What would its organic content be? At least we know a good spot from which to reach the water: the lower elevations of Herschel crater. All we need is a big drill rig—a really big one.

In the meantime, we’ll have to settle for fly-by views like this one, from the Cassini orbiter: